1. Fields of the Invention
The present invention is directed to a flow control valve, and more particularly to such a valve utilizing a shape memory alloy wire for varying a valve opening for regulation of a fluid flow.
2. Description of the Prior Art
U.S. Pat. No. 4,973,024 and Japanese Laid-Open Publication (KOKAI) No. 2-8573 disclose prior art flow control valves using a shape memory alloy wire for regulating a fluid flow. The valve comprises a barrel with an axial bore, a valve seat in the axial bore, and a valve body axially movable within the bore relative to the valve seat to vary a valve opening formed between the valve body and the valve seat. Formed within the barrel is an internal flow path extending from an inlet to an outlet through the valve opening. The shape memory alloy wire bridges between the valve body and the barrel and is connected to an electrical source in order to be heated in a controlled manner, undergoing thermal deformation along its length. The deformation or varying length of the wire causes the valve body to move against a spring bias in a direction of increasing or decreasing the valve opening for regulation of the fluid flow. In this known valve, the wire extends within the axial bore of the barrel with its opposite ends connected respectively to the valve body and the barrel and therefore must be connected to the electric source by way of the valve body as well as the barrel. Thus, the valve body and the barrel are required to be made of an electrically conductive material, which reduces design flexibility. In addition, the valve body and the barrel might form an additional electrical path other than through wire or might fail to establish a constant electrical path only through the wire not influenced by the movement of the valve body. In this event, the wire could be heated in an uncontrolled manner so as to result in a failure of reliably regulating the fluid flow. Further, since the wire extends through the internal fluid path, the wire will be thermally affected by the fluid passing through the path so that the wire suffers from unintended thermal deformation, which eventually leads to unreliable regulation of the fluid flow.
These problems could be eliminated in a flow control valve proposed in Japanese Laid-Open Publication (KOKAI) No. 61-103081. The valve comprises a barrel with an axial bore, a valve seat fixed in the axial bore, and a valve body axially movable within the barrel to vary a valve opening defined between the valve body and the valve seat. The barrel is formed with an inlet and an outlet which are communicated through the valve opening to establish an internal flow path within the barrel. A like shape memory alloy wire is provided for connection of the valve body to the barrel in order to vary the valve opening in response to a controlled heating of the wire. In this valve, the wire is wound round a portion of the valve body and is physically fixed at its opposite ends to the barrel, while the wire is electrically connected to an electrical source through terminals at the opposite ends of the wire. Thus, the wire can be directly connected to the electrical source without passing through the valve body and the barrel, thereby assuring reliable control of regulating the fluid flow by the use of the wire. However, there still remains a problem that the wire winding round the valve body within the axial bore adds an extra axial length to a structure of the valve body and the valve seat arranged axially along the barrel, which is a hindrance to give a fully-compacted design to the flow control valve using the shape memory alloy wire. This size problem becomes more critical as the length of the wire is required to be greater for obtaining an increased amount of linear deformation of the wire available for the control of the valve opening.